Film dosimetry is frequently used as a means for measuring 2-dimensional distributions of radiation dose. In the practice of medical radiotherapy, film dosimetry measurements are made for the quality assurance of radiation producing equipment as well as for the validation of therapeutic treatment plans prior to exposure of human patients to radiation sources.
A silver halide based dosimetry film responds to radiation exposure by developing a latent image. Upon development of the latent image a visual image is produced wherein the optical density at any point in the image is representative of the dose of radiation absorbed by the film. A radiochromic dosimetry film responds to radiation exposure by immediately developing a visible image wherein the optical density at any point in the image is representative of the dose of radiation absorbed by the film. One advantage of radiochromic film over silver halide film is that the former does not require post-exposure processing to develop a visible image.
One particular aspect of film dosimetry involves the measurement of a radiation-produced image in a film scanner. The film scanner produces a digital image of the dosimetry film. The digital image essentially describes the lightness or darkness of the film over an array of points and the film scanner is able to digitize the film image at high spatial resolution. While measurements of the film could be made with a manual densitometer, a scanner is the preferred method because it is able to measure a large area and make measurements at many points in a very short time period.
Film scanners of two basic types are in common usage. The first type employs a small beam of light to scan the film in a raster pattern. This type of scanner frequently uses a laser light source. The second type of scanner uses a large, diffuse light source to broadly illuminate the film and project an image of the film on a linear or 2-dimensional CCD array. Scanners of this second type are referred to as CCD scanners and are the ones most frequently used for film dosimetry. Representative examples are the Vidar VXR-16, Epson 1680 and the Microtek 9600XL.
GAFCHROMIC EBT dosimetry film is a radiochromic film used for film dosimetry. In measuring this film with CCD scanners it has been discovered that the measured response of the film is dependent on its position on the scanner. This parameter is characteristic of commercial CCD scanners such as Vidar VXR-16, Epson 1640XL, Epson 1680, Epson 4990, Epson V700, Epson 10000XL, Microtek 9800 and Microtek i900. In these scanners the light source is a linear fluorescent tube. What has been discovered is that the response of the EBT film varies with the position along the length of the light source. In general, the transmission of the film appears to be highest close to the center of the light source and lowest at the ends. In contrast, it has been found herein that when a piece of clear polyester is scanned on one of these scanners that the transmission of the film is essentially independent of position on the scanner.
In investigating these phenomena we have discovered that the variable response with the EBT film and other films that have a hazy appearance is at least in part a result of light scattering by the film. If a crystal-clear film is being measured, its response is independent of position, but if the film is hazy then the response varies with position. Since a crystal clear film does not scatter light, only certain light rays that are refracted through the film will be focused onto the CCD detector. The position-dependent response of a hazy film, for example GAFCHROMIC EBT dosimetry film, arises because, in addition to these refracted rays, a small proportion of other light rays that intersect the film over 2π space will be fortuitously scattered in the direction of the CCD array. Since the linear fluorescent light source has a finite length, the intensity of illumination at the center of the film will be greater than at the ends. Hence the contribution of scattered light to the total intensity of the image at the CCD detector will vary with position. Thus the transmission of a light-scattering film will appear to be highest at the center and lowest at the edges. Films that scatter light in this manner are referred to as light-scattering films.
The scanner output is proportional to the light intensity measured by the CCD. It is usual for the detected light intensity to be divided, over the dynamic range of the scanner, into 2n levels where n is an integer. Earlier digital film scanners had 28 levels and were commonly referred to as 8-bit scanners. Later models had 212 levels. At present most scanners have 16-bits of digitization. That is, they measure light intensity over a 216 dynamic range, i.e. there are 65536 levels between the highest and lowest intensities. Some scanners only have the ability to digitize an image in a single response channel. However, many scanners have the ability to produce digital images in multiple response channels. Thus a color scanner can provide a digitized image in three response channels, i.e., red, green and blue response channels. In addition these color scanners provide a grayscale response channel by combining the data in the three color response channels.
When digitizing a transparent film with a 16-bit film scanner, it is common to represent the highest intensity in the range by 65535 (216−1) and the lowest intensity (completely black) by 0. When scanning and measuring EBT dosimetry film in this way we discovered that we could make a first order correction for light scattering by scanning a piece of unexposed film, measuring the film response profile parallel to the linear light source, normalizing the response profile to the mean response and then using the normalized response profile to correct the positional variation in response of any other EBT film. This correction was found to work well, except when the film had become very dark because of high exposure to radiation, and/or the sides of the film were close to the edges of the scan area and hence were close to the ends of the linear light source.